Glutamic acid is a most popular excitatory neurotransmitter in the central nervous system, and the receptors thereof are classified broadly into an NMDA type, an α-amino-3-hydroxy-5-methyl-4-isoxazolepropionic (AMPA) type, a kainate type, and a metabotropic type. It is revealed that the NMDA type receptor plays an important role in the completion of the long-term potentiation (LTP), which is an electrophysiologically basal process of the memory/learning functions (cf., Science 285: 1870-1874 (1999)). At the animal level, it is known that an NMDA receptor antagonist may induce memory/learning dysfunctions in various memory/learning tasks such as a passive avoidance response, a radial maze, a T or Y maze, a water maze, a place or object recognition, an autoshaping learning task, and a lever-pressing task (cf., Brain Res Rev 41: 268-287 (2003)). It is also reported that PCP or ketamine, which is also an NMDA receptor antagonist, induces cognitive dysfunctions in humans (cf., Psychopharmacology 169: 215-233 (2003)). Namely, it is a common opinion among the electrophysiological level, the animal level and the human level that the NMDA type receptor plays an important rule in the memory/learning process.
It is the most widely-accepted hypothesis that the hypofunction of the NMDA type receptor is regarded as a mechanism of development of schizophrenia. This hypothesis is established based on the following four points:
(i) PCP and ketamine, which are an NMDA receptor antagonist, induce the major symptoms of schizophrenia in normal humans including cognitive dysfunction, positive negative symptoms, and induce the exacerbation of symptoms in patients with schizophrenia (cf., Am J Psychiatry 158: 1367-1377 (2001), Psychopharmacology 169: 215-233 (2003)):(ii) There are clinical reports that glycine, D-serine and D-cycloserine, which have been known to elevate NMDA receptor functions, enhance the drug efficacy of anti-psychotic drugs in patients with schizophrenia, and improve negative symptoms and cognitive dysfunctions (cf., Am J Psychiatry 158: 1367-1377 (2001), Psycho-pharmacology 169: 215-233 (2003)):(iii) The variation in the amount of glutamic acid per se, or the amount of the endogenous substance: N-acetyl-aspartyl glutamate (NAAG) having an NMDA antagonistic activity is observed in patients with schizophrenia (cf., Am J Psychiatry 158: 1367-1377 (2001)), and the change in the amount of mRNA and proteins of NMDA type receptor subunit and NMDA receptor-related protein is observed in patients with schizophrenia (cf., Am J Psychiatry 157: 1811-1823 (2000), Am J Psychiatry 160: 1100-1109 (2003), Society for Neuroscience Program No. 754.4. (2003)):(iv) A series of genes which were found as a gene relating to the onset of schizophrenia (Neuregulin 1, G72, dysbindin, calcineurin) are genes being capable of modifying NMDA receptor functions (cf., Proc Natl Acad Sci USA 99: 13365-13367 (2002), Neuron 40: 881-884 (2003)). From the above facts, it is generally considered that the dysfunctions induced by NMDA type receptor antagonists are models to reflect the dysfunctions by schizophrenia.
Schizophrenia is associated with various cognitive dysfunctions such as attention, memory, learning, executive functions, but it is reported that among these functions, especially a certain memory function is selectively and seriously damaged. Namely, memory is classified broadly into procedural memory and declarative memory. The declarative memory is further classified into short-term memory/working memory and a long-term memory/reference memory. It is reported that in schizophrenia, the declarative memory including both of the working memory and the reference memory is selectively damaged, and further, among them, the damage of the reference memory is most serious. Recently, it is reported that the cognitive dysfunction including such a reference memory dysfunction is the most important predictive factor of social daily ability and professional ability, and a quality of life of patients with schizophrenia. Then, at the moment, the cognitive dysfunction is positioned as a core symptom of schizophrenia (cf., Psychopharmacology 169: 213-214 (2003), Proc Natl Acad Sci USA 96: 13591-13593 (2002), Psychoneuroendocrinology 28: 27-38 (2003), Psychiat Clin N Am 26: 25-40 (2003)). Under these circumstances, many clinical trials are being done with respect to the effects of the existing anti-psychotic agents on various cognitive dysfunctions by schizophrenia (cf., Psychopharmacology 162: 11-17 (2002), Psychoneuroendocrinology 28: 27-38 (2003), Psychiat Clin N Am 26: 25-40 (2003)), and there is being submitted an evidence that a typical anti-psychotic agent, haloperidol, is ineffective, while some of atypical anti-psychotic agents are effective (cf., J. Clin Psychiatry 65: 361-372, Psychoneuroendocrinology 28: 27-38 (2003), Psychiat Clin N Am 26: 25-40 (2003)). However, the drug efficacy of these existing drugs are not sufficient enough, and hence, it has been discussed that it is important to develop a therapeutic agent for cognitive dysfunctions by schizophrenia (cf., Science 299: 350-351 (2003)).
On the other hand, in the research and development of therapeutic agents for human diseases, it is generally essential to develop an animal model being suitable for screening thereof. In such an animal model, the face validity (similarities of symptoms), the construct validity (similarities of the mechanism of development of symptoms), and the predictive validity (predictability of clinical drug efficacy), and further, the easiness being suitable for screening are required. However, it is considered that an animal model for cognitive dysfunctions by schizophrenia satisfying such requirements is quite limited at the moment. Namely, as an animal model for schizophrenia being capable of satisfying the above-mentioned requirements, PCP-induced models showing prepulse inhibition or social interaction failure can be exemplified (cf., Prog Neuropsychopharmacol Biol Psychiaty 27: 1071-1079 (2003)). It is known that the drug efficacy of atypical anti-psychotic agents, but that of a typical anti-psychotic agent can be selectively detected in these models, and the results obtained in these animal models partially reflect the clinical effects of a drug on schizophrenia. However, it is considered that among these animal models the former one may reflect the disorder of the sensorimotor gating function in schizophrenia, while the latter may reflect the negative symptoms of schizophrenia such a social withdrawal. Thus, as mentioned above, an easy model for evaluating drug efficacy, (1) being capable of reflecting reference memory dysfunction, which is the most serious cognitive dysfunction in schizophrenia (i.e., face validity, similarities of symptoms); (2) being associated with NMDA receptor hypofunction, which is a most possible cause for schizophrenia (i.e., construct validity, similarities of the mechanism of onset); and (3) being capable of detecting an excellent drug efficacy of an atypical anti-psychotic agent rather than that of a typical anti-psychotic agent (predictive validity, predictability of clinical drug efficacy), is considered to be quite useful in the research and development of a therapeutic agent for cognitive dysfunction, a core symptom of schizophrenia, but such an animal model has not been known yet until now.
In animals, various tasks consisting of both of the training session, and the testing session being carried out after a prescribed period from the testing session, can be used in order to study a reference memory. In the training session, the animals are made to learn an avoidance response such as electroconvulsive shock (passive or active avoidance response), a task of reaching to a platform which is not visible below water (a water maze task), a task of fetching a food after getting through a maze task or a task of avoiding an electroconvulsive shock (radial maze task, Y or T maze task), a task of recognizing and searching a novel place or object, a task of pressing a lever for obtaining a food, etc., and further they are made to acquire the memory thereof. The animals are returned to exactly the same experimental environment after a prescribed period therefrom, and they are tested if they can retrieve the acquired memory. In the reference memory tasks, the animals can memorize all of the specific environment and stimulus which are given to them in a specific order in the training session. In order to correctly evaluate at the testing session the success and failure of the memory acquisition in the training session, it is necessary to carry out the training session and the learning session under exactly the same environment and stimulus. Especially, there is a report that animals show an abnormal memory retrieval when ethanol or an NMDA type receptor antagonist is administered only in the training session of the reference memory tasks, but not in the testing session (cf., Brain Res 706: 227-232 (1996), Pharmcol Biochem Behav 69: 585-593 (2001)). In such cases, the animals acquire a reference memory depending on the environment in the brain, which is induced by the administration of a drug, and in fact, it is proved that by administering a drug both in the training session and the testing session, the acquired memory can correctly be retrieved. Such a phenomenon is usually called state-dependency. In the evaluation of the reference memory functions, it is sometimes necessary to give sufficient consideration to the state-dependency in some cases, and it is necessary to avoid any artificial misjudgment on the evaluation of memory functions due to state-dependency. As mentioned above, in order to avoid any state-dependency effect by an agent for inducing memory/learning dysfunctions such as an NMDA type receptor antagonist, and to evaluate the reference memory/learning function, there is a method comprising administering an agent for inducing memory/learning dysfunctions in both of the training session and the testing session. Further, as an alternative method, it is easily speculated to utilize a method comprising chronically administering an agent for inducing memory/learning dysfunctions during the period including the training session and the testing session. In addition, instead of administering an agent for inducing memory/learning dysfunctions, it may be possible to utilize a method comprising expressing a chronic dysfunction during the period including the training session and the testing sessions of the reference memory tasks with the use of a gene engineering technique. For example, an NMDA type receptor subtype NR1 knockdown or NR2A subtype knockout animal are already produced as a concrete example for an animal model showing a chronic hypofunction of NMDA type receptor (cf., Cell 98: 427-436 (1999), J Neurosci 21: 750-757 (2001)).